1. Field of the Invention
Glass powders can be used for various applications. One of these is the dental sector, in which plastic dental compositions are increasingly being used for dental restoration. These plastic dental compositions usually consist of a matrix of organic resins and various inorganic fillers. The inorganic fillers predominantly consist of powders of glasses, (glass-) ceramics, quartz or other crystalline substances (e.g. YbF3), sol-gel materials or Aerosils, and they are added to the plastic composition as filling material.
2. Description of Related Art
The use of plastic dental compositions is intended to avoid possible harmful side-effects of amalgam and to achieve an improved aesthetic impression. Depending on the plastic dental compositions selected, they can be used for different dental restoration measures, for example for tooth fillings and also for securing parts, such as crowns, bridges and inlays, onlays etc.
The filling material per se is intended to minimize the shrinkage caused by the polymerization of the resin matrix during curing. For example, if there is a strong adhesion between tooth wall and filling, excessive polymerization shrinkage can lead to the tooth wall breaking. If the adhesion is inadequate, excessive polymerization shrinkage may result in the formation of peripheral gaps between tooth wall and filling, which can promote secondary caries. Furthermore, certain physical and chemical demands are imposed on the fillers.
It must be possible to process the filling material to form powders that are as fine as possible. The finer the powder, the more homogeneous the appearance of the filling. At the same time, the polishing properties of the filling are improved, which in addition to reducing the surface area available for attack also leads to improved resistance to abrasion and therefore to a longer-lasting filling. To enable the powders to be processed successfully, it is also desirable for the powders not to agglomerate.
Furthermore, the refractive index and colour of the plastic dental composition in its entirety and therefore also of the filler should be as well matched as possible to the natural tooth material, so that it is as far as possible indistinguishable from the surrounding, healthy tooth material. The grain size of the pulverized filler being as small as possible also plays a role in this aesthetic criterion.
Further properties are desirable and known to a person skilled in the art and are described in more detail, for example, in DE 102009008954 B4.
Among plastic dental compositions, a distinction also needs to be drawn between dental cements and composites. In the case of dental cements, also known as glass ionomer cements, the chemical reaction of the fillers leads to curing of the dental composition, and consequently the curing properties of the dental composition and therefore the workability thereof are influenced by the reactivity of the fillers. This often involves a setting process which is preceded by a radical surface curing, for example under the action of UV light. Composites, also referred to as filling composites, contain, by contrast, fillers which are as chemically inert as possible, since their curing properties are determined by components of the resin matrix itself and a chemical reaction of the fillers is often disruptive for this.
Such a composite filling usually consists of a polymer matrix which, in order to improve the chemical and also the physical properties, can contain up to 90% by weight of an inert filler described. Common organic polymers are mostly dimethacrylates, for example triethylene glycol dimethacrylate (TEGDMA), UDMA, bis-GMA, acrylate, methacrylate, 2,2-bis-[4-(3-methacryloxy-2-hydroxypropoxy)phenyl]propane (bis-GMA), urethanemethacrylate and/or alkanediol dimethacrylate. However, it is also possible to use other polymer resins, e.g. epoxy-based resins, and the mixtures or copolymers thereof.
The plastics used in a plastic dental composition described usually have a refractive index in the range of 1.45 to 1.65. In order to achieve the highest possible transparency of the material, the refractive index of the fillers has to be matched to that of the surrounding matrix. Differences in refractive index between the polymer matrix and the fillers used, and also a difference between the fillers themselves, lead to scattering and reflection at the interfaces, and these result in reduced transparency. The finer the powder, the more homogeneous the appearance of the filling, and therefore also the higher the transparency.
A reduced transparency in the visible wavelength range has a negative effect on the optical appearance of the filling. A high transparency in the UV range is also important, in particular for the processing properties, however. The composite material is usually cured by irradiation with UV light. Common curing lamps have a wavelength range of 380 nm to 515 nm and/or 420 nm to 480 nm. A low transmissivity in the UV spectral range results in a relatively small depth of polymerization. A material with a high UV absorption has to be applied in a plurality of steps, in particular in the case of large layer thicknesses, in order to ensure sufficient polymerization. If this is not the case, the material can be cured in one step. For these reasons, it is advantageous if the filler, i.e. in particular the glass powder, has the smallest possible particle size, since the interaction in the visible and also in the ultraviolet wavelength range decreases with a falling particulate size of the glass powder.
In order to produce a glass powder which is suitable as a dental filler and has high transparency values and a mean particulate size in the micrometer or submicrometer range, special grinding processes are required. For the production of the glass powders by way of such fine grinding processes, use is made of stirred ball mills, vibration mills or bead mills. During grinding, it should be ensured that no disruptive contamination is formed by dust from the mill lining and/or the grinding beads used. Therefore, use is made of glass grinding beads which have matched refractive indices and consist of the same material or a material with the same refractive index as the glass to be ground to form the glass powder. The dust formed during the grinding thus has no negative effects on the optical properties of the filler. For the mill lining, use is made either of ceramics such as Zr2O3, Al2O3, SiC or plastics such as polyurethane. In the case of a ceramic lining, only minimal dust is formed. In the case of mills lined, for example, with polyurethane, the plastic dust can be removed again from the powder by a burn-out operation. Such grinding processes are described in detail, for example, in DE 4100604 C1 or EP 1005911 A1.
For the production of glass powders having particulate sizes in the micrometer or submicrometer range by grinding in vibration mills, stirred ball mills or bead mills, grinding beads having a diameter of less than 2 millimeters (mm) are usually used, in order to achieve effective comminution. The grinding beads are accelerated onto one another in the mill and comminute the filling material, i.e. the starting glass of the glass powder to be obtained, when they collide therewith. The grinding process produces a glass powder having particulates with a more or less sharp diameter distribution which generally resembles a Gaussian bell curve. The mean particulate size corresponds to the position of the maximum of the distribution.
In practice, however, it has been found that glass powders produced by the described grinding processes can repeatedly be found to have, in addition to the expected statistical distribution of the particulate size, particles with a significantly greater particle size, which are subject to a further statistical distribution. The statistical grain size distribution of the glass powder over all the components thereof (particulates and/or particles) therefore has at least two maxima in this case.
The diameter of these large particles may be dependent on the size of the grinding beads used, and, in the case of grinding beads having a diameter of less than 2 mm, is about 5 micrometers (μm) to 100 μm. The content of these particles is usually low and can be approximately 1 gram (g) or less per 20 kilograms (kg) of glass powder, but is very disruptive when the glass powder is used as a dental filler, because such particles have a disadvantageous effect on the gloss consistency and also the polishing properties of the plastic dental composition. Over the course of abrasion of the surface, the coarse particles then protrude from the surface of the plastic dental composition or form craters upon complete break out. These irregularities in the surface lead to reduced gloss or a reduced optical quality. The instances of break out can to some extent be identified as small holes with the naked eye. For these reasons, such large particles which fall out of the statistical distribution of the particulate size of the glass powder are undesirable in a glass powder.